Bowel cancer represents one frequent cause of death in the western nations. One good initial indicator of this illness is polyps which are formed in the bowel quite a long time before the outbreak of bowel cancer itself. In many cases, these develop over several years to form malignant carcinomas.
One effective measure for combating bowel cancer is thus early identification of polyps. Endoscopic methods have been used in the past for this purpose. Recently, the technique of virtual colonography has been becoming increasingly important, in which the bowel is scanned by way of computer tomography or magnetic resonance tomography and the images obtained in the process are then analyzed at medical workstations.
In the case of both techniques, colonoscopy by way of an endoscope and virtual colonography, the bowel must be cleaned by suitable medication before the examination, in order to allow structures which indicate polyps to be identified on the bowel wall. Patients consider this to be unpleasant, and this frequently forms an impediment to carrying out this examination.
In one known method for virtual colonography, the patient is fed with a low-fiber diet for one or two days before the examination, supplemented with a marking contrast agent which can be identified well in the imaging process. In this case, barium or gastrografin are predominantly used for computer tomography. The marking contrast agent mixes to a greater or lesser extent with stool or liquid residues in the bowel, and can thus be identified directly in the subsequently recorded medical images.
In order to achieve better contrast on the bowel wall during the examination, a so-called basic contrast agent (for example air or CO2 in the case of CT, water in the case of MR) is added. During post-processing of the image data, the stool marking contrast agent and liquid marking contrast agent mixtures are then identified using a threshold-value process, and are eliminated from the one or more images by digital subtraction. Bowel cleaning initiated by medication need no longer be carried out, or need be carried out only to a restricted extent, with this technique, which is also referred to as virtual bowel cleaning.
The high contrast difference between the air-filled bowel and the surrounding bowel wall is imaged by the imaging processes used onto a transition over a plurality of pixels or voxels in the 2D or 3D image produced in this case. This may be due to partial volume artifacts or to the transfer function. However, the subtraction process that is carried out for virtual bowel cleaning leads to imaging errors in particular at the transitions between areas which have marking contrast agent and the bowel walls, which have a similar structure to the polyps that are being looked for, thus making it difficult or even impossible to identify them.
An improved method for virtual bowel cleaning takes account of these transitions and uses a filter mask to carry out a different subtraction process within the areas filled with contrast agent and at the transition to the bowel wall. In this method, which is described in more detail for example in M. E. Zalis et al., “Digital Subtraction Bowel Cleansing for CT Colonography Using Morphological and Linear Filtration Methods” IEEE Transactions on Medical Imaging, Vol. 23, No. 11, 2004, pages 1335 to 1343, the transition to the bowel wall is modeled again after subtraction of the areas filled with marking contrast agent.
In this case, an edge detection process is carried out in each of the axial 2D image data records, in order to detect the sharp transitions that result after the subtraction process. These transitions are then replaced by a smoother falling transitional area, as can normally be seen in CT images at the transition between the bowel wall and the air-filled bowel. These newly modeled transitions are then low-pass filtered in order to carry out a final smoothing process. The entire method is carried out in two dimensions, that is to say with the individual 2D slice images.